 This activity produced a diverse sequence of volcanic deposits, well exposed in the bluffs along the Lillooet River, which is defined as the Pebble Creek Formation.The eruption was episodic, occurring from a vent on the north-east side of Plinth Peak.An unusual, thick apron of welded vitrophyric breccia may represent the explosive collapse of an early lava dome, depositing ash several meters (a dozen or so feet) in thickness near the vent area.